I work at an hourly rate. As a rule of thumb a typical full-colour illustration, including x2 rounds of feedback and revisions can take around 5 hours.
This can vary based on the brief, do you have a clear idea of what you needed, or do you want to harness the imagination of the illustrator?
I always provide initial sketches for any job, so we have something to look at and talk about. The best advice I can give anyone looking for illustration is to begin the conversation and flex the budget and time to get the best result.
Email me at firstname.lastname@example.org